Glass-ceramics are polycrystalline materials formed by controlled crystallization of a precursor glass article. A glass-ceramic may be prepared by exposing a glass monolith to a thermal treatment for conversion to a crystalline state. This is referred to as a “bulk” or “monolith glass-ceramic forming process.” U.S. Pat. No. 2,900,971 to Stookey describes the monolithic glass-ceramic forming technology. In general, raw materials that usually contain a nucleating agent are melted and simultaneously cooled to form a glass monolith of desired geometry. Subsequently, the glass monolith is exposed to a crystallizing thermal treatment that is referred to in the art as “ceramming.” The appropriate thermal treatment typically includes a low temperature hold somewhere about the transformation range to induce nucleation, followed by one or more temperatures holds at temperatures above the softening point of the glass to promote crystal growth. In the monolith forming process nucleation transpires internally. The manufacture of glass-ceramics by bulk forming processes is compatible with the high-speed, automated manufacturing processes that are employed in the formation and manufacture of glass articles. In addition, one advantage of internal nucleation is that it can provide a wide range of polycrystalline microstructures. Consequently, by tailoring the temperature treatment regimes, one can alter the properties of the final glass-ceramic material.
Glass-ceramics may also be prepared by firing glass frits in what is referred to as powder processing methods. A glass is reduced to a powder state (frit), formed to a desired shape, and then fired and crystallized to a glass-ceramic state. In this process, the surfaces of the glass grains serve as nucleating sites for the crystal phases. The glass composition, particle size, and processing conditions are chosen such that the glass softens prior to crystallization and undergoes viscous sintering to maximum density just before the crystallization process is complete. Shape forming methods may include but are not limited to extrusion, slip casting, tape casting, spray drying, and isostatic pressing.
Glass-ceramic materials have properties that may make them suitable for many other uses. A recent application for glass-ceramic materials, and one growing in importance, is as a sealing agent for solid oxide fuel cells (“SOFC”). SOFCs are basically energy reactor systems in which chemical energy is converted to electrical energy. While they are similar to batteries, the differ in that they do not run down as do batteries because SOFCs are continuously supplied with fuel and are thus able to continuously supply electricity. They are thus limited only by the available supply of fuel, the same as any normal power plant. SOFCs operate at high temperatures in the range of 600-1000° C., though current research is seeking to lower this temperature range, and use ceramic materials for functional elements of the cell.
As a general description, SOFC are composed of an anode and a cathode separated by a solid impermeable electrolyte that conducts oxygen ions from the cathode to the anode where they react chemically with the fuel. The electrical charge that is induced by the passage of the ions through the electrolyte is collected and conducted from the cell to the use source. While each cell generates only a limited voltage, a number of cells can be constructed in series to increase the voltage to a useful power level. Small SOFCs units in 5-10 kW are available from various corporations and larger units from 25-125 kW are under development or in testing throughout the world.
When designing SOFC it is important that the fuel (H2, CH4, C2H8, CO, etc) and air (O2) streams be kept separate and that thermal balance be maintained so that the operational temperature of the unit stays in an acceptable range. In order to ensure that both are accomplished, ceramic materials have been widely used in the design of SOFCs. However, since it is necessary to have a number of single cells in series in order to make a unit generating a usable amount of power, it is necessary that not only does the working components with a single cell be kept separate (no leakage), but also that there is no leakage from cell-to-cell in a series of cells. While various materials have been used as sealing agents, for example, epoxies and cements among others, improvements in this area are needed. The present invention discloses glass-ceramic materials that can be used as sealing materials.
The present invention is directed to seals for solid oxide fuel cells and novel compositions suitable for forming glass-ceramic sealing agents for such use.